U.S. patent number 6,143,366 [Application Number 09/220,824] was granted by the patent office on 2000-11-07 for high-pressure process for crystallization of ceramic films at low temperatures.
Invention is credited to Chung Hsin Lu.
United States Patent |
6,143,366 |
Lu |
November 7, 2000 |
High-pressure process for crystallization of ceramic films at low
temperatures
Abstract
A process is disclosed for reducing the crystallization
temperature of amorphous or partially crystallized ceramic films by
providing a higher pressure under which the crystallization of the
amorphous or partially crystallized ceramic films can be
significantly enhanced. The present invention not only improves
quality, performance and reliability of the ceramic films, but also
reduces the cost for production. By lowering the crystallization
temperature, the cost for thermal energy consumed during the
crystallization process is greatly reduced. In addition, the
interaction or interdiffusion occurring between films and
substrates is significantly suppressed or essentially prevented,
avoiding the off-stoichiometry and malfunction of thin films, which
usually occur in the conventional high-temperature crystallization
processes. The process of present invention also decreases the
grain size of formed films, thus reducing the roughness of films
and producing relatively smooth, good quality films. This process
made possible the fabrication of ceramic films with larger area at
substantially lower temperatures without using other excitation
energy such as laser, ion beam or electron beam, and is applicable
to very large scale integrated circuit technologies. The present
invention finds broad applications including manufacturing
electronic and optical devices such as ferroelectric memories,
capacitors, actuators, piezoelectric transducers, pyroelectric
sensors, gas-sensors, electro-optic displays, electro-optic
switching, non-liner optical devices, and reflective/antireflective
coating, etc.
Inventors: |
Lu; Chung Hsin (Taipei,
TW) |
Family
ID: |
22825128 |
Appl.
No.: |
09/220,824 |
Filed: |
December 24, 1998 |
Current U.S.
Class: |
427/376.3; 117/6;
117/7; 117/8; 204/192.15; 204/192.2; 205/224; 257/E21.272;
427/248.1; 427/376.1; 427/376.2; 427/377; 427/529; 427/596 |
Current CPC
Class: |
C23C
18/1208 (20130101); H01L 21/02197 (20130101); H01L
21/02266 (20130101); H01L 21/02282 (20130101); H01L
21/02337 (20130101); H01L 21/02356 (20130101); H01L
21/31691 (20130101); H01L 41/314 (20130101) |
Current International
Class: |
C23C
18/00 (20060101); C23C 18/12 (20060101); H01L
21/02 (20060101); H01L 21/316 (20060101); H01L
21/314 (20060101); B05D 003/02 (); C25C
005/50 () |
Field of
Search: |
;427/377,376.2,376.3,376.1,249.1,523,527,529,530,562,563,564,566,567,569,576,575
;204/192.15,192.16,192.2,192.22 ;117/6,7,8,9,10 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
000241179A3 |
|
Dec 1986 |
|
DE |
|
359018188 |
|
Jan 1984 |
|
JP |
|
360033285 |
|
Feb 1985 |
|
JP |
|
Other References
Lu, et al., "Laser-induced phase transformation from amorphous to
perovskite in PbZr.sub.0.44 Ti.sub.0.56 O.sub.3 films with the
substrate at room temperature," Appl. Phys. Lett., vol. 65, No. 17,
Oct. 1994. .
Yoshimura, et al., "In situ fabrication of morphology-controlled
advanced ceramic materials by Soft Solution Processing," Solid
State Ionics, vol. 98, No month 1997, pp. 197-208. .
Yu, et al., "Electron irradiation induced crystallization of
amorphous MgAl.sub.2 O.sub.4," Materials Chemistry and Physics,
vol. 46, (No month) 1996, pp. 161-165. .
Yu, et al., "High-quality epitaxial growth of .gamma.-alumina films
on .alpha.-alumina sapphire induced by ion-beam bombardment,"
Physical Review B, vol. 52, No. 24, Dec. 1995, pp.
17518-17522..
|
Primary Examiner: Padgett; Marianne
Attorney, Agent or Firm: Ladas & Parry
Claims
What is claimed is:
1. A method for producing a crystallized ceramic film comprising
the steps of:
(a) forming an amorphous or partially crystallized ceramic film on
a substrate; and
(b) crystallizing said amorphous or partially crystallized ceramic
film at an elevated temperature and under a pressure,
said temperature being below about 550.degree. C., said pressure
being larger than 10 atmospheres and lesser than about 250
atmospheres, said method being conducted within a chamber provided
with a species that remains volatilized at said temperature and
pressure, whereby said pressure is provided by a pressure of vapor
of said species.
2. The method of claim 1, wherein said amorphous or partially
crystallized ceramic film is crystallized within a closed
chamber.
3. The method of claim 1, wherein said species is selected from the
group consisting of oxygen, nitrogen, argon, hydrogen, water vapor,
carbon dioxide, organic solvent and inorganic solvent or mixtures
thereof.
4. The method of claim 3, wherein said species is selected from the
group consisting of oxygen, nitrogen, argon, hydrogen, carbon
dioxide and water vapor.
5. The method of claim 1, wherein said amorphous or partially
crystallized ceramic film is formed by chemical vapor deposition,
spin coating, dipping, evaporation, electroplating, electrophoretic
deposition, ion-beam deposition, sputtering, or laser ablation.
6. The method of claim 1, wherein said elevated temperature is
below 500.degree. C.
7. The method of claim 1, wherein said elevated temperature is
between 100.degree. C. and 400.degree. C.
8. The method of claim 1, wherein the crystallized ceramic film is
a ferroelectric ceramic film.
9. The method of claim 8, wherein said crystallized ceramic film is
selected from the group consisting of SrBi.sub.2 Ta.sub.2 O.sub.9,
BaBi.sub.2 Ta.sub.2 O.sub.9, (Sr.sub.x Ba.sub.1-x)Bi.sub.2 Ta.sub.2
O.sub.9, PbTiO.sub.3, and Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 wherein x
is from 0 to 1.
10. The method of claim 9, wherein said crystallized ceramic film
is SrBi.sub.2 Ta.sub.2 O.sub.9 and said species is selected from
the group consisting of oxygen, nitrogen, argon, hydrogen, water
vapor, carbon dioxide, organic solvent and inorganic solvent or
mixtures thereof.
11. The method of claim 9, wherein said crystallized ceramic film
is Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 wherein x is from 0 to 1 and said
species is selected from the group consisting of oxygen, nitrogen,
argon, hydrogen, water vapor, carbon dioxide, organic solvent and
inorganic solvent or mixtures thereof.
12. The method of claim 9, wherein said crystallized ceramic film
is PbTiO.sub.3 and said species is selected from the group
consisting of oxygen, nitrogen, argon, hydrogen, water vapor,
carbon dioxide, organic solvent and inorganic solvent or mixtures
thereof.
13. The method of claim 1, wherein said crystallized ceramic film
is a paraelectric ceramic film.
14. The method of claim 1, wherein said crystallized ceramic film
is Pb.sub.3 Nb.sub.4 O.sub.13.
15. The method of claim 1, wherein said crystallized ceramic film
has a crystal structure of perovskite, layered-perovskite,
pyrochlore, spinel, or fluorite.
16. The method of claim 1, wherein said substrate is selected from
the group consisting of silicon, ceramic, metal, and polymer
material.
17. The method of claim 1, further comprising the step of
re-heat-treating at a second temperature ranging from 100.degree.
C. to 600.degree. C.
18. The method of claim 1, wherein said pressure is within a range
from between about 20 atmospheres and about 250 atmospheres.
19. The method of claim 1, wherein said pressure is within a range
from between about 40 atmospheres and about 250 atmospheres.
Description
FIELD OF THE INVENTION
The present invention relates to an improvement in a process for
the fabrication of crystallized ceramic films. More specifically,
this invention relates to a process for crystallizing ceramic films
at relatively low temperatures by using high-pressure
environment.
BACKGROUND OF THE INVENTION
The wide range of interesting and useful properties exhibited by
ceramic films makes them technically important. The ceramic films
can be deposited or coated on objects to prevent the coated objects
from corrosion or attrition. On the other hand, the ceramic films
exhibiting specific electric properties constitute key components
in a wide range of electronic and optical devices. The electronic
devices using ceramic films include ferroelectric memories,
piezoelectric transducers, capacitors, dielectric resonators,
gas-sensors, pyroelectric sensors, actuators, and transducers.
Particularly, the ferroelectric ceramic thin films have
increasingly attracted great interest for the use in nonvolatile
random access memories (NVRAM) due to their large reversible
spontaneous polarization, and for the use in dynamic access
memories (DRAM) owing to their high dielectric constant. As for the
optical devices, ceramic films have been applied in non-liner
optical devices, electro-optic switching, electro-optic displays,
and reflective/antireflective coating.
To prepare ceramic films for different applications, various types
of processing methods have been developed. These methods can be
classified into two categories: namely, chemical and physical
processes. For chemical processes, chemical vapor deposition, spin
coating, dipping, and sol-gel processing have been developed. As
for the physical processes, evaporation, ion-beam deposition,
molecular beam epitaxy, radio-frequency and DC sputtering, and
laser ablation have been widely investigated. In general, the
as-grown ceramic films deposited on substrates at low temperatures
are amorphous or partially crystallized. In order to completely
crystallize the as-prepared films, either the high temperatures on
substrates during the deposition processes or the post-annealing
process at high temperatures for deposited films is required.
Usually, the crystallization process of ceramic films demands
fairly high temperatures. For example, the crystallization
temperature for Pb(Zr, Ti)O.sub.3 and SrTiO.sub.3 is 600.degree. C.
at least, and that for SrBi.sub.2 Ta.sub.2 O.sub.9 is above
650.degree. C. The high-temperature heating often results in
interference in the film-substrate interface, and causes
difficulties in integrating the films with substrates. The most
common substrates for ferroelectric ceramic films are silicon
substrates owing to their wide applications in ULSI technology.
During the high-temperature annealing processes, the silicon will
be oxidized to form silicon dioxide layers, bringing difficulties
in integrating the ceramic films with silicon monolithic circuits.
The high-temperature annealing also significantly enhances the
diffusion of the species into ceramic films as well as substrates,
thereby rendering the interdiffusion and interaction between films
and substrates. The undesirable reactions occurring in the
film-substrate interface result in the deviation of the composition
in ceramic films and doping of foreign atoms in silicon circuits
which cause serious problems in varying the electrical properties
of ceramic films as well as silicon circuits. The high-temperature
annealing also enhances the grain growth rate on films and results
in coarsening of grains and increases the roughness of films. The
rough morphology of films increases the difficulties in the
subsequent processes of etching and patterning. As a result, the
high temperature processing for preparing ceramic films is not
suitable to be integrated into the silicon processing technologies,
due to the temperature limitations as to the stability of the
underlying silicon wafers and structures and properties of ceramic
films.
In order to overcome the drawbacks of high-temperature processing
for crystallizing as-deposited ceramic films, new crystallization
processes such as hydrothermal processing, hydrothermal
electrochemical processing, laser annealing, ion-beam bombardment,
electron-irradiation, and plasma processing have been developed
recently. In hydrothermal processing and hydrothermal
electrochemical processing, substrates are immersed into aqueous
solutions containing constituent species and are reacted with
solutions to form the desired compounds on the substrate surface
(see, for example, Yoshimura et al., Solid State Ionics, Vol. 98
(3/4), p. 197 (1997)). The hydrothermal processes prepare various
ceramic films at relatively low temperatures ranging from
100.degree. C. to 300.degree. C. However, in these processes, the
substrates are immersed in reactive solutions and directly contact
with reactive solutions. Since the reactive solutions usually
contain high concentration of acid or alkaline reactants,
undesirable corrosion will easily occur on the substrates. As for
USLI technology, the silicon substrates usually have different
patterns or metal lines deposited on the surface of substrates. Due
to the corrosion problems, the hydrothermal processing is not
suitable for ULSI technology.
Laser-induced crystallization processes have been investigated
recently. Lu et al., Appl. Phys. Lett. Vol. 65 (16), p. 2015
(1994), used rf-magnetron sputtering to prepare PbZr.sub.0.44
Ti.sub.0.56 O.sub.3 (PZT) amorphous ceramic films on glass
substrates, then laser scanned the resultant films. The output
power of the laser-scan was 2.5 W, beam spot diameter was 90 .mu.m,
and the scanning rate was 4.5 cm/s. Under these conditions, the
amorphous PZT films can be transformed into crystallized state at
room temperature. Varshney, U.S. Pat. No. 5,626,670, also used
similar laser techniques to enhance the crystallization of PZT
films prepared by the spin-on sol-gel process.
On the other hand, Yu et. al., Phys. Rev. Vol. 52 (24), p. 17518
(1995), investigated the crystallization of alumina films induced
by ion-beam bombardment. Amorphous alumina films were coated on
single crystal alumina, and then bombarded with argon or oxygen
ions at temperatures ranging from 400.degree. C. to 600.degree. C.
This study indicates that the ion-beam bombardment effectively
induces the amorphous-to-.gamma. phase transformation of alumina.
Yu et. al., Mater. Chem. and Phys. Vol. 46 (2/3), p.161 (1996),
also employed electron irradiation to facilitate the
crystallization of amorphous MgAl.sub.2 O.sub.4 films. Single
crystal MgAl.sub.2 O.sub.4 substrates were coated with amorphous
MgAl.sub.2 O.sub.4 films by Xe ion irradiation. After the coated
films were subjected to electron irradiation at 300 keV at room
temperature, the crystallized MgAl.sub.2 O.sub.4 films were
obtained. Carl et al, U.S. Pat. No. 5,468,687, utilized the ozone
enhanced plasma to enhance the crystallization of Ta.sub.2 O.sub.5
films prepared by chemical vapor deposition, and reduced the
annealing temperature to as low as 400.degree. C.
Although the laser annealing, ion-beam bombardment, and
electron-irradiation techniques can successfully induce the
crystallization of amorphous ceramic films at relatively low
temperatures, the small beams of laser, ion, and electron beams
pose significant concerns when the above technologies are applied
to mass produce crystallized ceramic films having large surface
areas. Using the scanning technique during the irradiation
processes can enlarge the area of crystallized ceramic films;
however, the low scanning rate limits the throughput of ceramic
films, and the possibility of mass production. On the other hand,
the high excitation energy sources including laser, ion, electron
beams, and plasma will damage ceramic films and create defects in
films, thereby deteriorating the electrical properties of prepared
films. Thus, in order to overcome the disadvantages encountered in
the prior methods, a process that can crystallize as-deposited or
as-coated ceramic films at relatively low temperatures and is
practicable for mass production, would be highly desirable.
SUMMARY OF THE INVENTION
As a result of numerous experiments, the inventor of the present
invention unexpectedly found that using high-pressure treatment in
a closed chamber can significantly enhance the low-temperature
crystallization of as-deposited or as-coated ceramic films prepared
by chemical or physical methods. Crystallized ceramic films can be
obtained at temperature as low as 200.degree. C.-400.degree. C. by
employing the high-pressure treatment in the present invention. The
crystallization temperature in the present invention is several
hundred degrees centigrade lower than that in conventional
annealing processes.
According to one aspect of the subject invention, a method is
provided for producing the crystallized ceramic film by forming an
amorphous or partially crystallized ceramic film on a substrate and
then heat-treating the amorphous ceramic or partially crystallized
film at an elevated temperature and under a pressure higher than 5
atm.
According to another aspect of the subject invention, ceramic films
produced by the above method are also provided.
The pressure of the process may be realized by, for example, but
not limited to, undergoing the process in a closed chamber, within
which volatile species are present to provide the desired vapor
pressure under the elevated temperature. By varying volatile
materials and hence changing the vapor pressure, the heating
temperature of coated ceramic films can be adjusted.
Due to the advantage of lowered crystallization temperatures, the
thermal budget during the processing of ceramic films is
significantly reduced, so is the energy cost. Furthermore, the
interaction or interdiffusion between ceramic films and substrates
that commonly happens in conventional high-temperature processes is
also effectively suppressed, which prevents the off-stoichiometry
and malfunction of thin films. Since the crystallization process
occurs at low temperatures, the coarsening of grains (grain growth)
is also suppressed. Therefore smooth films having fine grains can
be obtained, and the roughness of films can be minimized. The
present invention does not require complicated and expensive
excitation energy such as laser, ion beam, electron beam, or
plasma. According to the present invention, only high-pressure
media are required for the crystallization of ceramic films at low
temperatures. This process is suitable for fabricating ceramic
films with large area, and is highly compatible with very
large-scale integrated circuit (VLSI) technologies. This invention
can be applied in fabricating electronic and optical devices such
as ferroelectric memories, capacitors, pyroelectric sensors,
gas-sensors, actuators, piezoeletric transducers, electro-optic
displays, electro-optic switching, non-liner optical devices, and
reflective/antireflective coating, etc.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates the x-ray diffraction of crystallized SrBi.sub.2
Ta.sub.2 O.sub.9 films prepared by a high-pressure treatment of
78.9 atm at 300.degree. C. for 2 hours according to the
invention.
FIG. 2 illustrates the x-ray diffraction of amorphous SrBi.sub.2
Ta.sub.2 O.sub.9 films heated at 300.degree. C. for 2 hours in a
conventional heating process.
FIG. 3 illustrates the x-ray diffraction of crystallized SrBi.sub.2
Ta.sub.2 O.sub.9 films prepared by a high-pressure treatment of
89.4 atm using ammonia solution as a pressure source at 300.degree.
C. for 2 hours according to the invention.
FIG. 4 illustrates the x-ray diffraction of crystallized SrBi.sub.2
Ta.sub.2 O.sub.9 films prepared by radio-frequency magnetron
sputtering and a subsequent high pressure treatment of 56.5 atm at
280.degree. C. for 2 hours.
FIG. 5 illustrates the x-ray diffraction of Pb(Zr.sub.0.52
Ti.sub.0.48)O.sub.3 films prepared by a high-pressure treatment of
77.6 atm at 300.degree. C. for 4 hours according to the
invention.
FIG. 6 illustrates the x-ray diffraction of amorphous
Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3 films heated at 300.degree. C.
for 4 hours in a conventional heating process.
FIG. 7 illustrates the x-ray diffraction of crystallized
PbTiO.sub.3 films prepared by a high-pressure treatment of 77.6 atm
at 300.degree. C. for 2 hours according to the invention.
FIG. 8 illustrates the x-ray diffraction of crystallized Pb.sub.3
Nb.sub.4 O.sub.13 films prepared by a high-pressure treatment of
77.6 atm at 300.degree. C. for 2 hours according to the
invention.
FIG. 9 illustrates the x-ray diffraction of amorphous Pb.sub.3
Nb.sub.4 O.sub.13 films heated at 300.degree. C. for 2 hours in a
conventional heating process.
DETAILED DESCRIPTION OF THE INVENTION
This invention is based on the inventor's unanticipated
experimental results. The inventor surprisingly found that using
high pressure remarkably induces the as-deposited or as-coated
ceramic film with amorphous or partially crystallized structure to
be well crystallized at substantially lower temperatures. Thus,
this invention is directed to a method for producing crystallized
ceramic film by forming an amorphous or partially crystallized
ceramic film on a substrate and then heat-treating the amorphous
ceramic or partially crystallized film at an elevated temperature
under a pressure higher than 5 atm.
The amorphous or partially crystallized ceramic film, or
equivalently called the as-deposited or as-coated ceramic film, is
prepared by chemical or physical methods, and then heated in a
closed chamber with high vapor pressure for inducing the
crystallization of the film. To the inventor's knowledge, this kind
of high-pressure treatment has never been reported in the past for
crystallizing as-deposited or as-prepared ceramic film.
Ceramics are inorganic nonmetallic materials which consist of
metallic and nonmetallic elements bonded together primarily by
ionic and/or covalent bonds. The chemical compositions of ceramics
vary considerably from simple compounds to mixtures of many complex
phases bonded together (see Smith, "Principles of Materials Science
and Engineering," second edition, p.559, McGraw-Hill Publishing
Company, New York, 1990). The types of ceramics include oxides,
nitrides, borides, carbides, halides, hydrides, or oxtnitrides. The
structures of ceramics include rock salt, zinc blende, perovskite,
complex perovskite, layered perovskite, pyrochlore, wurtzite,
corudum, illmenite, rutile, spinel, anti-spinel, olivine, fluorite,
antiflourite, cesium chloride type, gibbsite, tungsten bronze, and
silicate type. Some ceramics contain the mixtures of the above
structures. See Kingery et al, "Introduction to Ceramics," John
Wiely & Sons, New York, 1991, and Galasso, "Structure and
Properties of Inorganic Solids," Pergamon, New York, pp. 39-251,
1970.
The ceramic films of the present invention are not limited to any
specific composition or crystal structure. It was found as
evidenced by the examples that the high-pressure process of the
present invention can enhance the crystallization of the amorphous
or partially crystallized ceramic films at relatively lower
temperatures, regardless of the composition or crystal structure of
the ceramic films.
The most common crystal structures for the ceramic films
contemplated in the present invention include simple cubic,
face-centered cubic, rock salt, zinc blende, perovskite, complex
perovskite, layered perovskite, pyrochlore, wurtzite, corudum,
illmenite, rutile, spinel, anti-spinel, olivine, fluorite,
antiflourite, cesium chloride type, gibbsite, tungsten bronze,
silicate type, and any mixture thereof. Ceramic materials with a
perovskite structure preferably has a composition of A(B',
B")O.sub.3, which are composed of the site A comprising at least
one element from lead, barium, strontium, calcium, and lanthanum,
bismuth, potassium and sodium, the sites B' and B" comprising at
least one element from magnesium, chromium, nickel, manganese,
iron, cobalt, copper, titanium, tin, zirconium, cerium, niobium,
molybdenum and tungsten.
As for the composition of the ceramic films, the present invention
is not limited to ceramic films of any specific composition.
Typical compositions of the ceramic films which are suitable for
the present invention are, but not limited to, SrBi.sub.2 Ta.sub.2
O.sub.9, BaBi.sub.2 Ta.sub.2 O.sub.9, and (Sr, Ba)Bi.sub.2 Ta.sub.2
O.sub.9 of layered perovskite structure, Pb(Zr, Ti)O.sub.3 and
PbTiO.sub.3 of perovskite structure, and Sr.sub.2 Ta.sub.2 O.sub.7
and Pb.sub.3 Nb.sub.4 O.sub.13 of pyrochlore structure. The
synthesized films of SrBi.sub.2 Ta.sub.2 O.sub.9, BaBi.sub.2
Ta.sub.2 O.sub.9, (Sr, Ba)Bi.sub.2 Ta.sub.2 O.sub.9, Pb(Zr,
Ti)O.sub.3, and PbTiO.sub.3 are all important ferroelectric
materials.
As evidenced by the examples in the specification, the
applicability of the present invention is not to be limited by the
structure and composition of ceramic films. The ceramic films
produced according to the present invention can be used in
fabricating DRAM, NVRAM, superconductor devices, conductive
electrodes, capacitors, flat displays, sensors, solar cells,
lithium batteries, and catalysis, and non-liner optical devices,
etc.
Compared with other prior processes in which the amorphous or
partially crystallized ceramic films are heated under ambient
pressure, the present invention undergoes the crystallization step
under elevated pressures. It is believed that any pressure higher
than atmospheric pressure can facilitate the crystallization and
the pressure is preferably higher than 5 atm, more preferably
between 10 atm and 250 atm.
By elevating the pressure in the heat-treatment step, the
crystallization can be enhanced and thus the temperature in
heat-treatment can be reduced, avoiding the problems as described
in the Background of the Invention section of the present
specification.
According to the present invention, the crystallization temperature
can be reduced to below 600.degree. C., preferably 550.degree. C.,
more preferably below 500.degree. C., most preferably between
100.degree. C. and 400.degree. C.
The amorphous or partially crystallized ceramic films may be formed
by any conventional chemical or physical processes which include
chemical vapor deposition, spin coating, dipping, sol-gel
processing, evaporation, electroplating, electrophoretic
deposition, ion-beam deposition, sputtering, sputtering, and laser
ablation.
The crystallization step of the present invention does not require
expensive and complicated equipments such as laser, ion/electron
irradiation, or plasma processes, and can be applied to fabricate
crystallized ceramic films with large areas. Therefore, the present
invention is advantageous for mass production. In one example as
described below, with a pressure of 56.5 atm, the crystallization
temperature of SrBi.sub.2 Ta.sub.2 O.sub.9 is as low as 280.degree.
C. which is the lowest temperature in the present art. As for
Pb(Zr, Ti)O.sub.3, the crystallization temperature can be lowered
to 300.degree. C. under a pressure of 77.6 atm. The low-temperature
process can reduce the consumed heating energy, and also makes the
ceramic films to be compatible with present ULSI technologies for
fabricating DRAM and NVRAM. Owing to the low-temperature heating,
the oxidation of silicon or metal can be reduced. According to the
analysis of secondary ion mass spectroscopy (SIMS), the interaction
and interdiffusion usually occurring at high temperatures in the
interface between films and substrates are significantly minimized.
Based on the analysis of scanning electron microscopy (SEM), the
grain size of formed films in the present invention is relatively
smaller than that prepared in high-temperature processing. Atomic
force microscopy (AFM) also confirms that the rather smooth films
are obtained in this invention. Consequently, the
high-pressure-enhanced crystallization process not only results in
suppression of interaction or interdiffusion between films and
substrates, but also improves the morphology of crystallized films
to have fine grains and smooth surface.
As for the substrates of ceramic films, any material that has been
known in the present art for forming ceramic films can be used.
Especially, it is possible to use materials having low melting
points, such as glass or polymers, since the heat treatment of
as-prepared ceramic films can be performed at relatively low
temperature.
The substrates contemplated by the present invention are not
limited to those having a flat surface. The substrates having
curved surfaces are also applicable in the present invention.
As for the volatile species for providing elevated vapor pressure,
all species which can be vaporized at the desired crystallization
temperatures and generate sufficiently high vapor pressure can be
used. In the cases of nitride or carbide ceramic films, the inert
gas such as nitrogen or argon is preferred for avoiding the
oxidation of prepared films. For using solution as the pressure
sources in the chamber, volatile species, for example, organic or
inorganic solvents such as benzene, ethyl alcohol, and acetone,
organic or inorganic alkali such as ammonium hydroxide,
tetraethylammonium hydroxide, and monoethanolamine, organic or
inorganic acids such as acetic acid, nitric acid, and formic acid,
can be added in the solution. Once the temperature is raised, the
added species vaporize to increase the vapor pressure. In case of
using solution as pressure sources at high temperature, the amount
of solution used should be sufficient to generate the required
pressure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be illustrated in greater detail by way
of the following examples. The examples are given for illustration
of the invention, and are not intended to be limited thereof.
EXAMPLE 1
This example shows the effects of high-pressure treatment on the
crystallization of layered perovskite-SrBi.sub.2 Ta.sub.2 O.sub.9
films.
The precursors of SrBi.sub.2 Ta.sub.2 O.sub.9 films were prepared
by mixing strontium 2-ethylhexanoate, bismuth 2-ethylhexanoate, and
tantalum ethoxide in stoichiometric proportion. The prepared
precursors were deposited onto a Pt/Ti/SiO.sub.2 /Si substrates by
spin-coating. The Ti layer was used to improve adhesion between Pt
and SiO.sub.2 layers. The coated films were dried at 150.degree.
C., and subsequently pyrolyzed at 350.degree. C. for around 10 min.
No diffraction peaks except for the peaks belonging to platinum
from the substrates were found in the pyrolyzed films by x-ray
diffraction (XRD) analysis. Thus, the pyrolyzed films were
identified to be amorphous. Then, these as-pyrolyzed films were
placed and heated in a closed bomb where water was filled in the
bottom of the bomb and used as the vapor source. The amount of
water was kept from direct contact with the films at the beginning
of the experiment. The heating temperatures were 260.degree. C. and
300.degree. C., and the duration time was 2 hours. The generated
pressures at the above two temperatures were 700 psi (47.6 atm) and
1160 psi (78.9 atm), respectively. After the high-pressure process
at 260.degree. C., the amorphous ceramic films became slightly
crystallized. Upon heating at 300.degree. C., well-developed
crystallized SrBi.sub.2 Ta.sub.2 O.sub.9 films were obtained as
shown in FIG. 1. In this figure, the diffraction peaks with (115),
(200), and (2010) indexes belonging to SrBi.sub.2 Ta.sub.2 O.sub.9
are clearly identified. Therefore, the effect of high-pressure
treatment on the crystallization of SrBi.sub.2 Ta.sub.2 O.sub.9
films was confirmed. The platinum diffraction peaks in FIG. 1 were
produced from the underlying substrates. The top platinum
electrodes were sputtered on the surface of prepared films, and a
standard ferroelectricity analyzer (RT66A) was employed to analyze
the ferroelectric characteristics. It was found that the SrBi.sub.2
Ta.sub.2 O.sub.9 films had a typical ferroelectric hysteresis
polarization-electric field (P-E) loop. In addition, in the fatigue
endurance test, no degradation in polarization occurred after
10.sup.9 switching cycles. The grain size of SrBi.sub.2 Ta.sub.2
O.sub.9 films was about 0.04 .mu.m as observed by scanning electron
microscopy (SEM). Accordingly, the prepared SrBi.sub.2 Ta.sub.2
O.sub.9 films exhibited excellent ferroelectric properties, and can
be applied in nonvolatile random access memories (NVRAM).
COMPARATIVE EXAMPLE 1
The same procedure as in Example 1 was repeated except that the
pyrolyzed films were heated in a conventional electric furnace,
instead of a closed chamber, at 300.degree. C. and 700.degree. C.
for 2 hours. It was found that no crystallization of SrBi.sub.2
Ta.sub.2 O.sub.9 films occurred after heating at 300.degree. C. as
shown in FIG. 2. On the other hand, increasing the heating
temperature to be as high as 700.degree. C. in the conventional
process resulted in the similar degree of crystallization in
SrBi.sub.2 Ta.sub.2 O.sub.9 films as that prepared in Example 1.
Therefore, the crystallization temperature of SrBi.sub.2 Ta.sub.2
O.sub.9 films in the high-pressure process is 400.degree. C. lower
than that in the conventional heating process. It definitely
reveals that the high-pressure process remarkably reduces the
crystallization temperature of SrBi.sub.2 Ta.sub.2 O.sub.9 films.
The grain size as observed by scanning electron microscopy (SEM)
was about 0.06 .mu.m, which is greater than the grain sizes of
SrBi.sub.2 Ta.sub.2 O.sub.9 films in Example 1. In addition, atomic
force microscopy (AFM) also confirmed that rougher surface was
obtained when compared with that of SrBi.sub.2 Ta.sub.2 O.sub.9
films in Example 1 because of its large grain size. According to
the analysis of secondary ion mass spectroscopy (SIMS), the high
temperature heating (700.degree. C.) in the COMPARATIVE EXAMPLE 1
resulted in the diffusion of bismuth species into the substrate
region. On the other hand, as shown in the results of the
low-temperature heating (300.degree. C.) process in Example 1, the
diffusion of bismuth species into substrates was suppressed.
Conclusively, the low crystallization temperature in the
high-pressure process significantly reduced the thermal budget in
processing as well as energy consumption. Furthermore, the
interaction or interdiffusion between films and substrates was also
greatly suppressed or prevented, avoiding the off-stoichiometry of
thin films and malfunction of substrates or wafers. Therefore, it
is significantly advantageous to integrate SrBi.sub.2 Ta.sub.2
O.sub.9 films with the present processes for the production of
silicon-based semiconductors. In addition, the high-pressure
process decreases the grain size of formed SrBi.sub.2 Ta.sub.2
O.sub.9 films, thereby reducing the roughness of films. The
production of relatively smooth SrBi.sub.2 Ta.sub.2 O.sub.9 films
is beneficial for the etching and patterning processes in ULSI
technologies.
EXAMPLE 2
This example shows the effects of vapor media on the
crystallization of SrBi.sub.2 Ta.sub.2 O.sub.9 films. For
increasing the applied pressure on prepared films, ammonia solution
with pH=12 was used in the high-pressure process to produce the
vapor species. Since at elevated temperatures ammonia will be
vaporized from the solution, the pressure in the closed chamber
will be increased. The same procedure as in Example 1 was repeated
except that the pyrolyzed films were heated in the chamber where
ammonia solution was used as the vapor source. Under heating at
300.degree. C., the pressure in the chamber was increased to be
1315 psi (89.4 atm) which was higher than that in Example 1. The
heating time was held for 2 hours. At the end of heating process,
it was found that the crystallized SrBi.sub.2 Ta.sub.2 O.sub.9
films were also formed (as shown in FIG. 3). The formed films
exhibited higher diffraction intensity than those prepared in
Example 1. This result reveals that better crystallinity of
SrBi.sub.2 Ta.sub.2 O.sub.9 films was achieved in Example 2.
Therefore, the high pressure produced by the vapor media was
confirmed to enhance the crystallization process of SrBi.sub.2
Ta.sub.2 O.sub.9 films. The enhanced crystallization of SrBi.sub.2
Ta.sub.2 O.sub.9 films in this example is attributed to the
increased vapor pressure by ammonia.
EXAMPLE 3
This example shows the effects of high-pressure treatment on the
crystallization of SrBi.sub.2 Ta.sub.2 O.sub.9 films prepared by
radio-frequency (rf) sputtering.
The SrBi.sub.2 Ta.sub.2 O.sub.9 films were deposited onto
Pt/Ti/SiO.sub.2 /Si substrates using a radio-frequency magnetron
sputtering system. The composition of the targets was
Sr:Bi:Ta=2:4:2 with excess strontium and bismuth in order to
compensate for the loss of Sr and Bi during the deposition process.
The distance between the substrates and targets was 3 cm, and the
sputtering time was 30 min. The as-deposited films were then
subjected to a high-pressure treatment of 830 psi (56.5 atm) at
280.degree. C. for 2 hours. The pressure was the same as that in
Example 1. After the high-pressure treatment, well crystallized
SrBi.sub.2 Ta.sub.2 O.sub.9 films were obtained. As shown in FIG.
4, this example confirmed that the high-pressure treatment
successfully induced the crystallization of rf-sputtered SrBi.sub.2
Ta.sub.2 O.sub.9 films. This example reveals that the high-pressure
treatment can enhance the crystallization of SrBi.sub.2 Ta.sub.2
O.sub.9 films no matter they are prepared by chemical processing
(such as Example 1) or physical processing (such as Example 3).
EXAMPLE 4
This example shows the effects of high pressure treatment on the
crystallization of ferroelectric layer perovskite-(Sr.sub.0.5
Ba.sub.0.5)Bi.sub.2 Ta.sub.2 O.sub.9 films.
The precursors of (Sr.sub.0.5 Ba.sub.0.5)Bi.sub.2 Ta.sub.2 O.sub.9
films were prepared by mixing barium 2-ethylhexanoate, strontium
2-ethylhexanoate, bismuth 2-ethylhexanoate, and tantalum ethoxide
according to the stoichiometric proportion. The prepared precursor
was deposited onto Pt/Ti/SiO.sub.2 /Si substrates using
spin-coating method. The coated films were dried at 150.degree. C.,
and subsequently pyrolyzed at 350.degree. C. for around 10 min. No
diffraction peaks except for the peaks belonging to platinum from
the substrates were found in the pyrolyzed films by x-ray
diffraction (XRD) analysis. Thus, the pyrolyzed films were
identified to be amorphous. Then, these as-pyrolyzed films were
placed and heated in a closed bomb in the similar conditions as
described in Example 1. The heating temperature was 280.degree. C.,
and the duration time was 2 hours. The generated pressures were 830
psi. After heating at 280.degree. C., well-developed crystallized
(Sr.sub.0.5 Ba.sub.0.5)Bi.sub.2 Ta.sub.2 O.sub.9 films were
obtained. In addition, the XRD pattern is similar to that of
SrBi.sub.2 Ta.sub.2 O.sub.9, indicating that the SrBi.sub.2
Ta.sub.2 O.sub.9 -based solid solution was formed. Therefore, the
high-pressure treatment is confirmed to be highly effective for
crystallizing the films of the solid solutions of SrBi.sub.2
Ta.sub.2 O.sub.9.
EXAMPLE 5
This example shows the effects of high pressure treatment on the
crystallization of ferroelectric Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3
films with a perovskite structure.
The precursors of Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3 films were
prepared by mixing lead 2-ethyhexanoate, zirconium n-propoxide, and
tetraisopropyl orthotitanate according to the stoichiometric
proportion. The prepared precursors were deposited onto
Pt/Ti/SiO.sub.2 /Si substrates using spin-coating method. The
coating and pyrolyzing processes were the same as described above.
The pyrolyzed amorphous ceramic films were subjected to a
high-pressure treatment. The heating condition was 300.degree. C.
under 1140 psi (77.6 atm), and the heating time was 4 hours. After
the above treatment, well crystallized Pb(Zr.sub.0.52
Ti.sub.0.48)O.sub.3 films were obtained, as shown in FIG. 5. In
this figure, the diffraction peaks with (100), (110), (111), and
(200) indexes belonging to Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3 are
clearly identified. This example confirms the availability of
applying the high pressure process to induce the crystallization of
Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3 films at low temperatures.
COMPARATIVE EXAMPLE 5
The same procedure as in Example 5 was repeated except that the
pyrolyzed films were heated in a conventional electric furnace
instead of a high-pressure chamber, at 300.degree. C. for 4 hours.
It was found that no crystallization of Pb(Zr.sub.0.52
Ti.sub.0.48)O.sub.3 films occurred after 300.degree. C.-heating.
The XRD pattern for the 300.degree. C.-annnealed films is
illustrated in FIG. 6. This figure shows that the annealed films
remain amorphous. In comparison with Example 5, the results
definitely reveal that the high-pressure process remarkably reduces
the crystallization temperature of Pb(Zr.sub.0.52
Ti.sub.0.48)O.sub.3 films.
EXAMPLE 6
This example shows the effects of high-pressure treatment on the
crystallization of PbTiO.sub.3 films with a perovskite
structure.
The precursors of PbTiO.sub.3 films were prepared by mixing lead
2-ethyhexanoate and tetraisopropyl orthotitanate in stoichiometric
proportion. The prepared precursor was deposited onto
Pt/Ti/SiO.sub.2 /Si substrates by spin-coating. The coated films
were dried at 150.degree. C., and subsequently pyrolyzed at
350.degree. C. for 10 min. The pyrolyzed amorphous ceramic films
were subjected to a high-pressure treatment. The heating condition
was 300.degree. C. under 1140 psi (77.6 atm), and the heating time
was 2 hours. As confirmed by the x-ray diffraction pattern of
PbTiO.sub.3 films, at the end of the above treatment, the well
crystallized PbTiO.sub.3 films having a perovskite structure were
obtained as shown in FIG. 7. In this figure, the diffraction peaks
with (001), (100), (101), (110), (002), and (200) indexes belonging
to PbTiO.sub.3 are clearly identified. This example confirmed that
the high-pressure treatment enhanced the crystallization of
perovskite PbTiO.sub.3 films at low temperatures. The fabricated
films are useful for the applications of ferroelectric memories,
piezoelectric devices and actuators.
EXAMPLE 7
This example shows the effects of high-pressure treatment on the
crystallization of paraelectric Pb.sub.3 Nb.sub.4 O.sub.13 films
with a pyrochlore structure.
Precursors of Pb.sub.3 Nb.sub.4 O.sub.13 films were prepared by
mixing lead 2-ethyhexanoate and niobium ethoxide according to the
stoichiometric proportion. The prepared precursor was deposited
onto Pt/Ti/SiO.sub.2 /Si substrates using spin-coating method. The
coating and pyrolyzing processes were the same as described above.
The pyrolyzed amorphous ceramic films were subjected to a
high-pressure treatment. The heating condition was 300.degree. C.
under 1140 psi (77.6 atm), and the heating time was 2 hours. After
the above treatment, well crystallized Pb.sub.3 Nb.sub.4 O.sub.13
films were obtained, as shown in FIG. 8. In this figure, the
diffraction peaks with (222), (400), (331), (422), (511), and (440)
indexes belonging to Pb.sub.3 Nb.sub.4 O.sub.13 are clearly
identified. This example confirms the effectiveness of applying the
high-pressure process to induce the crystallization of Pb.sub.3
Nb.sub.4 O.sub.13 films at low temperatures.
COMPARATIVE EXAMPLE 7
The same procedure as in Example 7 was repeated except that the
pyrolyzed films were heated in a conventional electric furnace,
instead of a high-pressure chamber, at 300.degree. C. for 2 hours.
It was found that no crystallization of Pb.sub.3 Nb.sub.4 O.sub.13
films occurred after 300.degree. C.-heating. The XRD pattern for
the 300.degree. C.-annnealed films is illustrated in FIG. 9. This
figure reveals that the annealed films remain amorphous. In
comparison with Example 7, the results definitely reveal that the
high-pressure process remarkably reduces the crystallization
temperature of Pb.sub.3 Nb.sub.4 O.sub.13 films.
EXAMPLE 8
This example shows the effects of high-pressure treatment on the
crystallization of paraelectric Pb.sub.3 Nb.sub.4 O.sub.13 films
prepared by the dipping process. Precursors of Pb.sub.3 Nb.sub.4
O.sub.13 films were prepared according to the procedure described
in Example 7. The prepared precursors were deposited onto
Pt/Ti/SiO.sub.2 /Si substrates using the dipping method at a rate
of 0.3 cm/sec. The coating and pyrolyzing processes were the same
as in Example 7, and only amorphous ceramic films were obtained.
These amorphous ceramic films were subjected to a high-pressure
treatment under 1140 psi (77.6 atm) at 300.degree. C. for 2 hours.
After the above treatment, well-crystallized Pb.sub.3 Nb.sub.4
O.sub.13 films were obtained. This example confirms that the high
pressure treatment can enhance the crystallization of Pb.sub.3
Nb.sub.4 O.sub.13 films no matter they are prepared by what types
of chemical processing such as spin-coating in Example 7 or
dip-coating in Example 8.
* * * * *